I. Field
The present invention relates generally to communication, and more specifically to a modulator for a transmitter in a wireless communication system.
II. Background
In a wireless communication system, a transmitter first digitally processes traffic/packet data to obtain coded data. The transmitter then modulates a carrier signal with the coded data to obtain a modulated signal that is more suitable for transmission via a wireless channel. The modulation may be performed based on various modulation schemes such as, for example, continuous phase modulation (CPM) and phase-shift keying (PSK).
With CPM, the phase of the carrier signal is modulated by the coded data in a continuous rather than abrupt manner. As a result, a CPM modulated signal (or simply, a CPM signal) has several desirable characteristics including a constant envelope and a compact spectrum. The constant envelope allows the CPM signal to be transmitted using an efficient power amplifier. The compact spectrum enables efficient utilization of the available frequency spectrum. However, the CPM signal also has a fairly complex waveform that can complicate the transmitter and receiver designs.
With PSK, the phase of the carrier signal is modulated by the coded data in an abrupt manner at each symbol period. Furthermore, the phase of the carrier signal is restricted to be at specific points in a signal constellation. Some commonly used PSK schemes include binary-PSK (BPSK), quadrature-PSK (QPSK), and 8-ary PSK (8PSK). PSK is also widely used because of the ease in which a PSK modulated signal (or simply, a PSK signal) may be generated at a transmitter and processed at a receiver.
A transmitter may be required to support multiple modulation schemes. For example, a transmitter may need to support Gaussian minimum shift keying (GMSK) for Global System for Mobile Communication (GSM) and 8PSK for Enhanced General Packet Radio Service (EGPRS). GSM is a wireless communication system that is widely deployed throughout the world. EGPRS is a packet data service for GSM and employs 8PSK to achieve a higher data rate. GMSK is one class of CPM, and a GMSK waveform has the complexity normally associated with a CPM waveform. Since GSM and EGPRS are widely deployed, it is highly desirable to have an efficient modulator that can support both GMSK and 8PSK.